Thermit welding chemistry: In the thermit process, which mixture reacts exothermically to produce molten metal for joining?

Introduction / Context:
Thermit welding relies on an aluminothermic reaction that generates intense heat and molten metal without external power sources. It is widely used for joining railway rails and heavy sections where conventional arc access is difficult.

Given Data / Assumptions:

• Thermit mixture is reacted in a refractory crucible above the joint.
• The reaction must be highly exothermic to melt steel and filler.
• Fluxes and ignition powder (e.g., barium peroxide/magnesium) may be used to initiate.

Concept / Approach:
The classic aluminothermic reaction uses finely divided aluminium as a strong reducing agent to reduce iron oxide to metallic iron while forming aluminium oxide. The reaction releases large quantities of heat, producing molten iron/steel which is directed into a prepared mould around the joint gap.

Step-by-Step Solution:
Choose reagents: Fe2O3 (or Fe3O4) + Al as fuel/reductant.Reaction (plain text): iron oxide + aluminium → aluminium oxide + iron + heat.The molten iron fills the mould and fuses the components after solidification.Therefore, the correct mixture is iron oxide and aluminium.

Verification / Alternative check:
Field practice and handbooks specify thermit as iron oxide and aluminium; charcoal is not part of the classical mixture.

Why Other Options Are Wrong:
Charcoal (carbon) does not deliver the required reaction rate/temperature in this setup.

Adding charcoal to the standard mix is unnecessary and detrimental.

Magnesium is used sometimes as an igniter, not the main reductant.

Common Pitfalls:
Incorrect proportions leading to incomplete reduction; poor preheat of the joint mould; inadequate gating causing shrinkage cavities.

Final Answer:
iron oxide and aluminium